JP2017106113A - Nitrided plate component and method for producing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a nitrided plate component exhibiting the fatigue strength equal to or above that of a carburization-treated material, and a method for producing the same.SOLUTION: Provided is a nitrided plate component having prescribed components and structure, characterized in that the nitrogen average content in which the distance to a shear end face normal direction from the shear end face of the component toward the inside of the nitrided plate component lies in the range of 0.05 to 0.10 mm being 0.4000 to 1.2000, and also, the lowest nitrogen content of 0.015 to 0.200 mm is 0.0600% or higher. After the coiling-opening of a steel sheet coil, tension of 0.03 to 3.00% by a plastic strain amount and compressive deformation are mutually applied to the surface layer of the steel sheet, thereafter, without re-coiling the steel sheet, it is subjected to shearing and press molding into a plate component shape, and subsequently nitrification is performed under prescribed conditions.SELECTED DRAWING: Figure 4

Description

  The present invention relates to an appropriate material manufacturing method and a nitrided plate part having excellent durability by performing gas soft nitriding after forming and a manufacturing method thereof, for example, a torque converter plate part and a manufacturing method thereof. .

  Many parts subjected to surface hardening treatment are used in automobiles and machine parts. The surface hardening treatment is generally performed for the purpose of improving wear resistance and fatigue strength, and typical surface hardening treatment methods include carburizing, nitriding, induction hardening, and the like.

  Unlike other methods, nitriding such as gas nitriding, gas soft nitriding, and salt bath soft nitriding has the advantage that heat treatment strain can be reduced. Therefore, nitriding is a surface hardening process suitable for parts that require precision processing after hardening, such as parts that have been subjected to precision machining, such as crankshafts and transmission gears, or disks and plates formed by pressing.

  Among the nitriding treatments, there are gas nitriding, salt bath nitriding, etc. Among them, gas soft nitriding treatment that is performed in a bath or atmosphere containing carbon together with nitrogen can be performed in a short time by increasing the nitriding potential. Well, it is possible to obtain a component having an increased surface hardened layer depth within a few hours. This gas soft nitriding treatment is characterized by the formation of a hardened surface layer with a high depth of hardened surface, which provides excellent wear resistance as a part, and the durability is greatly improved by the effect of surface hardening. is there. From the above, gas soft nitriding is a technology that combines excellent dimensional accuracy, wear resistance, and economy, and carburizing and quenching for the purpose of improving wear resistance is replaced by gas soft nitriding. Is required.

  However, gas soft nitriding parts using steel materials as raw materials need to be processed in a temperature range of A1 point or less in order to form a surface compound layer having excellent wear resistance. As a result, martensite transformation does not occur as in carburizing and induction hardening processes, and in general, the residual stress of compression generated in the surface of the component is small, and it is difficult to ensure durability equal to or higher than that of the carburized material. .

  The plate component that plays the role of power transmission that constitutes the torque converter has a claw portion disposed on the side surface of the plate joined to the turbine, and transmits power via a spring disposed on the piston. At this time, the claw portion is loaded in the in-plane direction of the plate, stress concentrates near the corner between the plate and the claw portion, and a fatigue crack is likely to occur from this portion. Component durability is improved by reducing the stress generated during power transmission. As the means, the shape of the corner portion between the plate and the claw portion can be loosened and the thickness can be increased, but it is not preferable from the viewpoint of space restriction and power transmission efficiency.

  On the other hand, Patent Document 1 discloses a technique for improving the fatigue strength after gas soft nitriding. In the technique disclosed in Patent Document 1, fatigue characteristics are improved by controlling the dislocation density and metal structure of the steel sheet.

  Moreover, the plate part for torque converters that plays the role of power transmission is generally subjected to a shearing process in the manufacturing process and then has a predetermined part shape through a pressing process. For this reason, the final product also receives the properties of the fracture surface generated during shearing. Regardless of the gas soft nitrided torque converter plate component, the end face has a high roughness and receives a microscopic stress concentration, resulting in a higher stress.

  For example, in order to improve the characteristics of the shear surface, Patent Document 2 invents a steel plate for a plate disk clutch. In Patent Document 3, there is an invention related to a steel plate material in which the durability of the shear end face is improved by controlling the dislocation density of the material, both of which are very effective techniques in applications where fatigue cracks are likely to occur from the shear surface. It is.

International Publication No. 2015/190618 JP 2001-73073 A International Publication No. 2013/077298

  However, the technique disclosed in Patent Document 1 is a technique for improving the fatigue characteristics of the planar portion, and is not a technique for achieving sufficient fatigue strength even when applied to a nitride plate component. This is because the fatigue strength of the nitride plate component is determined by the durability of the shear end face. In addition, in the steel sheet having a component containing Ti or Nb as described in Patent Document 1, setting the ferrite fraction to 80% or more is a cause of reducing the fatigue strength in the plane portion of the nitride plate component. Become. That is, yield elongation occurs in ferritic steel containing Ti and Nb. This yield elongation is a cause of forming a wrinkle pattern on the surface of the pressed part before the nitriding treatment. Since this wrinkle pattern causes stress concentration, the fatigue strength of the surface other than the shear end face is reduced. Further, when there is a shear end face, the fatigue strength of the shear end face is remarkably reduced by concentrating the stress microscopically at the shear end face and the surface ridge line of the pressed part.

  In addition, as will be described later, in the study of the present inventor, the technique described in Patent Document 3 is not a technique that can be applied to develop the durability of a torque converter component that has been subjected to gas soft nitriding that is equal to or higher than the carburized material. I understood. This is because the torque converter component subjected to gas soft nitriding does not start from the shear end face but cracks from the inside in the vicinity of the shear end face. In Patent Document 3, the fatigue strength of the shear end face is evaluated by a plane bending fatigue test having a punched hole. In the plane bending fatigue test with the punched hole, the edge of the shear end face of the punched hole (that is, the ridge line formed between the steel plate surface and the shear end face) receives the highest stress. However, since the nitride plate parts are uniformly loaded in the plane of the shear end face, the fatigue crack generation behavior is different from the plane bending fatigue test with punched holes. Therefore, the technique described in Patent Document 3 cannot sufficiently increase the fatigue strength of the nitride plate component.

  Accordingly, an object of the present invention has been made in view of the above problems, and it is an object of the present invention to provide a nitride plate component that exhibits fatigue strength equal to or higher than that of a carburized material and a method for manufacturing the same.

  The present inventors arranged the characteristics of the position where the fatigue crack occurred from the inside of the part near the shear end face by various factors. As a result, nitride plate parts represented by torque converter plate parts subjected to gas soft nitriding (hereinafter referred to simply as “nitride plate parts” or “plate parts” The fatigue strength is also well-correlated with the fatigue crack initiation position. Furthermore, by controlling the nitrogen concentration inside the part to a predetermined value, the fatigue strength equivalent to or higher than that of the carburized material should be exhibited. I found. As a result of further investigations, the fatigue crack initiation position can be controlled by the shearing strain history of the part, which means that the fatigue strength is improved by limiting the chemical composition and manufacturing conditions of the material to a specific range. I understood. From these studies, we succeeded in making the fatigue strength of nitrided plate parts, which seemed to be difficult to control, equal to or greater than the fatigue strength of carburized materials (hereinafter sometimes referred to as “carburized plate parts”). The present invention has been achieved. The specific means is as follows.

(1) A nitride plate component having a shear end face,
The chemical composition excluding nitrogen is mass%,
C: 0.025% or more, 0.113% or less,
Si: 0.10% or less,
Mn: 0.71% or more, 1.49% or less P: 0.020% or less,
S: 0.0200% or less,
Ti: 0.020% or more, 0.091% or less,
Cr: 0.130% or more, 0.340% or less,
Al: 0.10% or more, 0.35% or less,
Nb: 0.001% or more and 0.020% or less,
Mo: 0.001% or more and 0.140% or less,
V: 0.001% to 0.100%,
B: 0.0001% or more and 0.0030% or less,
Cu: 0.00% or more and 0.13% or less,
Ni: 0.00% or more and less than 0.08%,
W: 0.00% or more and 0.07% or less,
Co: 0.00% or more and 0.07% or less,
Ca: 0.0000% or more and less than 0.0070%,
Mg: 0.0000% or more and less than 0.0050%,
In the nitride plate component containing REM: 0.0000% or more and less than 0.0050%, and the balance: Fe and impurities, and the area ratio of the ferrite structure in the metal structure is 70% or less,
The nitrogen average content in the range of 0.05 mm or more and 0.10 mm or less in the distance from the shear end face to the shear end face normal direction of the nitride plate part is 0.4000% or more and 1.2000% or less in mass%, and The minimum nitrogen content of 0.015 mm or more and 0.200 mm or less is 0.0600% or more,
Nitrogen content in the region where no nitrogen intrusion occurs due to nitriding treatment, the origin is the thickness direction center of the nitride plate part at least 5 mm away from the shear end face in the normal direction of the shear end face, and the thickness center line from the origin A nitrogen plate component in which the nitrogen content is measured on a line at 3 to 5 points in a section up to a position 1 mm away along the line, and the average value of the nitrogen content is 0.0007% or more and 0.0300% or less.

(2) The chemical composition is mass%,
C: 0.025% or more, 0.113% or less,
Si: 0.10% or less,
Mn: 0.71% or more, 1.49% or less P: 0.020% or less,
S: 0.0200% or less,
Ti: 0.020% or more, 0.091% or less,
Cr: 0.130% or more, 0.340% or less,
Al: 0.10% or more, 0.35% or less,
N: 0.0007% or more, 0.0100% or less,
Nb: 0.001% or more and 0.020% or less,
Mo: 0.001% or more and 0.140% or less,
V: 0.001% to 0.100%,
B: 0.0001% or more and 0.0030% or less,
Cu: 0.00% or more and 0.13% or less,
Ni: 0.00% or more and less than 0.08%,
W: 0.00% or more and 0.07% or less,
Co: 0.00% or more and 0.07% or less,
Ca: 0.0000% or more and less than 0.0070%,
Mg: 0.0000% or more and less than 0.0050%,
REM: 0.0000% or more and less than 0.0050%, and balance: slab containing Fe and impurities is hot rolled in a range of hot rolling finish rolling outlet temperature of 850 ° C. or more and less than 960 ° C. Get
Thereafter, cooling is started within 3 seconds after the hot rolling finish rolling is completed, and further, the steel sheet is cooled to 460 ° C. or more and 630 ° C. or less within 29 seconds after the hot rolling finish rolling is finished,
By winding up the steel sheet, the area ratio of the ferrite structure in the metal structure is a steel sheet coil of 70% or less,
Furthermore, in the case of manufacturing a nitride plate part using the pickled steel plate coil, after unrolling the steel plate coil, the steel plate has a plastic strain amount of 0.03% or more and 3.00% or less. Of bending and unbending,
Without re-rolling the steel plate again, it is subjected to shearing and press forming into a plate part shape,
A nitride plate in which the steel sheet is allowed to stay in a closed furnace adjusted to a temperature of 500 ° C. or higher and lower than 620 ° C. for 60 minutes or longer in an atmosphere having a volume composition ratio of ammonia gas of more than 30% and nitrided. A manufacturing method for parts.

  According to the present invention, it is possible to develop a fatigue strength equal to or higher than that of the carburized material by controlling the occurrence position of the fatigue crack of the nitride plate part, which has been difficult to control conventionally. This makes remarkable industrial contributions, such as the ability to manufacture parts that balance the economics of part performance.

It is a microscope picture which shows the cross section of the measurement site | part of R components. It is a microscope picture which shows the cross section of the measurement site | part of R components. It is a front view which shows the test piece of a fatigue test. It is a graph which shows the relationship between N ^** and a fatigue crack generating position. It is an image which shows the SEM observation result of a fatigue fracture surface. It is a graph which shows the relationship between nitrogen average content and fatigue strength. It is a graph which shows the relationship between N ^* , N ^** and fatigue strength. It is a graph which shows the influence of the plastic strain amount which acts on N ^* . It is a graph which shows the influence of the plastic strain which acts on N ^** .

  Exemplary embodiments of the present invention will be described below in detail with reference to the accompanying drawings.

  Nitride plate parts represented by torque converter plate parts are arranged perpendicular to the rotation axis, and as a result, are subjected to stress outside the shear plane of the plate, but not from the shear end face but from inside the shear end face. A fatigue crack occurs. The present invention focuses on the occurrence position of this fatigue crack, investigates the relationship with fatigue strength, limits the nitrogen content of nitrided parts, and the average chemical composition of the parts. The reason for the limitation will be described below. To do.

1.1 Nitrogen Content of Nitrided Part First, the background of focusing on the nitrogen content and the reason for limiting the nitrogen content will be described. Here, the nitrogen content in the present invention is measured with an EPMA (Electron Probe Micro Analyzer) apparatus, and a value identified from a Kα ray reflected from an electron beam irradiated from a W filament is adopted. In addition, although the gas measurement etc. are mentioned as another measuring method of nitrogen, since spatial resolution is scarce, it is not preferable as a measuring method.

Further, since nitrogen is generated when the surface of the surface such as oil is decomposed by an electron beam, the surface finish of the measurement object is important. As the surface finishing method, cut the measurement surface, mirror finish with fine particles such as emery paper and alumina, and then perform ultrasonic cleaning in a liquid such as acetone or ethanol without corrosion by nital etc. After drying with a blower or the like, it is necessary to perform measurement after drying in a closed container containing silica gel for at least 24 hours. The sealed container is preferably connected to a rotary pump or the like, and the degree of vacuum is preferably increased to about 10 ⁻³ Torr.

  Since the gas soft nitriding is performed in a closed furnace adjusted to an atmosphere described later, the surface in contact with the furnace atmosphere is uniformly nitrided. Therefore, the measurement location of nitrogen content may be selected from the shear end face of any part of the nitriding plate part and cut in the vertical direction from the shear end face to the inside of the plate part. Surface finishing may be performed and the nitrogen content may be measured. However, since the surface other than the shear end face is also nitrided, the measurement position in the direction of the shear end face is 0.001 mm along a line within a range of ± 0.1 mm from the center of the plate thickness as a position not affected by this. As described above, the nitrogen content may be measured at intervals of 0.005 mm or less.

  Here, as for the nitrogen content, an arbitrary shear end face of the part may be selected, but it is sufficient to perform measurement at least at three locations and measure at most five locations in consideration of measurement variation. In addition, it is preferable to perform a fatigue test in advance and include a position where a fatigue crack occurs as a measurement location.

  Based on the nitrogen content line data obtained by the measurement method described above, interval integration was performed in the range from 0.05 mm to 0.10 mm in the distance from the shear end surface to the normal direction of the shear end surface for reasons described later. A value obtained by dividing the total amount of nitrogen content obtained by dividing by the number of measurement points in the section is defined as the nitrogen average content of the nitride plate component, which is defined as the nitrogen average content of the component portion.

  The minimum nitrogen content in which the distance from the shear end face to the inside of the nitride plate part in the normal direction to the shear end face is 0.015 mm or more and 0.200 mm or less is defined as a value measured as follows. That is, as a position that is not affected by nitrogen intrusion from a surface other than the shear end face, the origin is a shear end face within a range of ± 0.1 mm from the center of the plate thickness, and the inside of the plate part in the normal direction of the shear end face is 0. In the linear data of nitrogen content measured at intervals of 0.001 mm or more and 0.005 mm or less, an average value of three points including a certain measurement point and its adjacent points is obtained. The lowest value in the range where the average value is 0.015 mm or more and 0.200 mm or less is called the minimum nitrogen content.

  Here, when measuring the minimum nitrogen content, the range of the measurement is because the nitride compound layer is formed when the distance from the shear end face to the inside of the plate part in the normal direction of the shear end face is less than 0.015 mm. It is necessary to exclude from. The sample for measuring the minimum nitrogen content may select an arbitrary shear end face of the component for the nitrogen content. In addition, the sample for measuring the minimum nitrogen content is the same as that measured for the average nitrogen content in the range of 0.05 mm or more and 0.10 mm or less in the distance from the shear end surface to the normal direction of the shear end surface. It may be used. However, in consideration of measurement variation, it is necessary to perform measurement at least at three locations, and it is sufficient to perform measurement at five locations at the maximum. In the present invention, the average value of the minimum nitrogen content at each location measured by the above method is directed from the shear end face toward the inside of the plate part, and the distance in the normal direction of the shear end face is 0.015 mm or more, 0. It is defined as the minimum nitrogen content of 200 mm or less.

  Further, the nitrogen content in the central part in the thickness direction at least 5 mm away from the shear end face may be obtained by the following measurement method as a region where no nitrogen intrusion is caused by gas soft nitriding. That is, in an arbitrary shear end face of the nitride plate part, the center in the thickness direction of the portion at least 5 mm or more away from the shear end face in the normal direction is the origin, and from the origin to a position 1 mm away along the thickness center line. The nitrogen content is measured on a line at an arbitrary interval such as 0.001 mm or more and 0.005 mm or less, and the average value of the nitrogen content in the section is obtained. The average value is measured at three arbitrary shear end faces of the nitride plate part, the average value is obtained, and the average value is determined by the nitrogen content at the center in the plate thickness direction at least 5 mm away from the shear end face. To do. Note that three or more measurement points may be measured. However, if a maximum of five points are measured, the measurement variation can be sufficiently ignored.

  Since the maximum penetration depth of nitrogen by gas soft nitriding is 0.6 mm at the maximum, in the nitride plate component having a plate thickness of less than 1.2 mm, the influence of nitrogen that has entered from the front and back surfaces can be produced. In the central part of the plate thickness, there is a large amount of segregation during the steel plate production stage, which becomes the starting point of cracks when the shear end face of the nitride plate part is deformed out of plane and broken brittlely. Therefore, the nitrogen content at the central portion in the thickness direction at least 5 mm away from the shear end face does not affect the fatigue intended by the present invention, but as a basic requirement of the nitride plate component, as described later. A range was defined. Note that the present invention is not limited to the above thickness, and even if the plate thickness is 1.2 mm or less, the nitrogen content in the central portion in the plate thickness direction at least 5 mm or more away from the shear plane is mass. In the case of 0.0007% or more and 0.0300% or less, the influence of nitriding is considered to be small, and it corresponds to the “nitrogen content in a region where nitrogen does not enter due to nitriding”. Further, as described later, since the ladle analysis value of the steel material component and the EPMA analysis value are substantially the same value, the ladle analysis value of the steel material component may be used. It is also possible to use the emission spectral statement analysis method of JIS G1258. The plate thickness of the nitride plate component is not particularly limited, but the plate thickness range may be 1.0 mm or greater and 8.0 mm or less. If necessary, the lower limit of the plate thickness may be 1.2 mm or 1.5 mm. The upper limit of the plate thickness may be 6.0 mm, 5.0 mm, or 3.8 mm.

  Note that gas soft nitriding often involves processing a plurality of identical parts in the furnace, and if it is assumed that the furnace is affected by the atmosphere such as gas stagnation in the furnace, it should be placed in the outermost position in the furnace. Extract one or more from each of the arranged parts and the part arranged in the center, measure the nitrogen content of the above nitrided plate parts, and whether all of the extracted nitrided plate parts reach the target value Can be determined. In the present invention, when the term “average nitrogen content” is simply used, it means the average nitrogen content of the above-mentioned nitride plate component.

  For reference, examples of measurement parts of R-shaped parts are shown in FIGS. FIG. 1 is an enlarged photograph of an R portion of a nitride plate part having an R shape, and FIG. 2 is an enlarged photograph of a cut surface of the cut portion described in FIG. As shown in FIG. 1, the shear end face of the R part is cut in the normal direction of its ridgeline, and the plate is set with the shear end face as 0 point in the range of the thickness center of the cross section (cut face shown in FIG. 2) ± 0.1 mm. The average nitrogen content may be measured in the range of 0.05 mm or more and 0.10 mm or less in depth in the internal direction, and the minimum nitrogen content in the range of 0.015 mm or more and 0.200 mm or less may be measured. In addition, although the cross-sectional photograph of FIG. 2 performs nital corrosion for easy viewing, it is not possible to perform corrosion as described above when measuring with EPMA. Moreover, the white site | part observed in all the surface layers in FIG. 2 is a nitride compound layer, and is excluded from a measurement range.

  Fatigue strength was evaluated by the following method to determine pass / fail. That is, for example, when a plate component used in a torque converter or the like plays a role of power transmission, torque is applied to the plate surface because it is arranged perpendicular to the rotation axis. At this time, the highest stress is applied to the plate shear end face. For the purpose of reproducing such a load state, a fatigue test simulating out-of-plane deformation was performed using the test piece shown in FIG. The plate thickness clearance control value in the pressing process is generally 15%, but the plate thickness clearance is 20% on the assumption that inferior shear end face properties will be caused by die wear and shaft misalignment. As shown in FIG. Here, the clearance in the shearing process refers to a gap between the punch or blade and the die during the shearing process. Further, the plate thickness clearance is a value obtained by dividing the clearance by the plate thickness.

The fatigue test was performed by applying a repeated load with a frequency of 25 Hz and a stress ratio of −1, and the stress amplitude of 10 ⁷ times was obtained from the SN curve. In general, the fatigue amplitude of 10 ⁷ times may be referred to as a fatigue limit, but in the present invention, it is referred to as fatigue strength. As the stress value, a value obtained by attaching a strain gauge so as to be parallel to the tangent to the R portion at the position of “applying a strain gauge” in FIG. 3 was adopted. The distance from the shear end face to the fatigue crack initiation position was measured by applying a stress amplitude 20 MPa higher than the fatigue strength and observing the fatigue fracture surface obtained by fatigue fracture with a scanning electron microscope (SEM). This is the value in the normal direction from the fatigue crack initiation position to the shear end face.

Incidentally, the fatigue fracture test conditions for measuring the occurrence position of the fatigue crack, if the stress amplitude number of cycles to failure is greater than or equal to 10 ⁵ times, may be selected what stress amplitude value. This is because the stress amplitude at which the number of repetitions of fracture is 10 ⁵ times or more is less than the yield stress, so that the shape of the DUT does not change during the fatigue test, and the fatigue crack generation position does not change depending on the stress amplitude. .

  The object of the present invention is to develop fatigue strength equal to or higher than that of the carburized material. Therefore, first, the fatigue strength of the target carburized plate reference part is obtained. Using the component described as Base in Table 1, the atmosphere was adjusted to a range of 0.8 to 0.9% by mass with a carbon potential of a press product created by the manufacturing method described in Table 2, and 910 ° C. After maintaining the temperature for 270 minutes, the carburized plate reference part was manufactured by oil cooling, and the fatigue strength was 517 MPa. Thereafter, the pass / fail of fatigue strength was determined using this value as a threshold value.

Furthermore, test numbers 1 to 22 in Table 3 are nitride plate parts that were prototyped by the manufacturing method described in Table 2 using the components of Try 1 to 11 in Table 1 (the ladle analysis values of the components of the steel material). The fatigue test results are compared with the fatigue strength of the carburized plate reference part. Except for Test Nos. 12, 13, 17, and 21, when each steel sheet coil was opened, it was a portion with a wave shape in the width direction, and thus it was difficult to perform shearing. Therefore, after the coil was unwound, a bending / unbending correction process to obtain a predetermined plastic strain amount was performed, and a nitride plate part was prototyped. In this step, as described later, the distance from the shear end face to the shear end face normal direction is N ^** which is the minimum nitrogen content in the range of 0.015 mm or more and 0.200 mm or less, and the shear end face to the shear end face normal. It has a close relationship with N ^* , which is the average nitrogen content in the range of 0.05 mm or more and 0.10 mm or less in the distance in the direction. In the following, the background and reason for limiting N ^* and N ^** as requirements will be described first, and then the relationship between N ^* and N ^** and the manufacturing method will be described.

  In addition, "N" content in Table 1 shows the quantity contained in a slab or a slab. In each example, the balance is iron and unanalyzed impurities. In Tables 2 and 3, “FT” is the hot rolling finish rolling temperature (° C.), “t1” is the time (seconds) from the end of hot rolling finish to the start of cooling, and “CT” is The cooling stop temperature (° C.), “t2” is the time (seconds) from the end of hot rolling finish rolling to the end of cooling (cooling stop), “d” is the fatigue crack initiation depth (mm), Show.

  As a result of the inventor's investigation, when the distance from the shear end surface to the fatigue crack generation position (hereinafter simply referred to as the fatigue crack generation position) is 0.200 mm or more, the fatigue strength of the nitride plate part is less than that of the carburized plate part. Some cases were exceeded. This is considered that the fatigue strength was satisfied because the fatigue crack generation position was deepened and the load stress was reduced. In the nitride plate parts, nitrogen is fixed to dislocations, thereby increasing the fatigue crack initiation limit stress. Therefore, in order to make the crack generation position more than 0.200 mm, it was examined whether it could be solved by adjusting the nitrogen amount at 0.200 mm or less.

FIG. 4 shows the relationship between the minimum value N ^** of the nitrogen content and the fatigue crack initiation position when the distance from the shear end surface to the normal direction of the shear end surface is 0.200 mm or less. 4 indicates that the fatigue strength was less than the carburized plate reference part, and ○ indicates that the carburized plate reference part was equal to or greater. According to FIG. 4, the fatigue crack generation position is uniquely determined by N ^** , and it is found that if the value is 0.0600% or more by mass, the fatigue crack generation position can be controlled to 0.200 mm or more. It has been found that setting N ^** to 0.0600% or more by mass is one of the requirements for satisfying the fatigue strength.

Furthermore, the requirements for satisfying fatigue strength were investigated. In the nitride plate part, a fatigue crack is generated from the inside in the vicinity of the shear end face. When a fatigue crack occurs inside, it can be confirmed that the fatigue crack has only occurred after it has propagated to the free surface. Therefore, the propagation resistance of fatigue cracks may also affect the fatigue strength. Therefore, the inventor made N ^*, which is an average nitrogen content in the vicinity of the shear end face, that is, the distance from the shear end face to the normal direction of the shear end face in the range of 0.05 mm or more and 0.10 mm or less within a certain limited range. We examined whether the fatigue strength could be improved by controlling.

Below, the examination result of N ^* is described.

SEM observation of the fatigue fracture surface was performed in the range from the shear end face to the fatigue crack initiation position for those in which a difference in fatigue strength occurred while satisfying N ^** . For SEM observation, the test number 6 with the highest fatigue strength among the satisfied fatigue strengths, the test number 20 that satisfied N ^** but did not satisfy the fatigue strength slightly, and the test number with a fatigue strength of 530 MPa Four nitride plate parts were selected. Each fatigue fracture test was performed under the following conditions. The nitride plate part produced as a trial with test number 6 was loaded with 583 MPa at a stress amplitude σa and fatigue-ruptured at 1.73 × 10 ⁶ times. Further, the nitride plate part prototyped with test number 20 was subjected to fatigue fracture at 2.65 × 10 ⁵ times with a stress amplitude σa of 534 MPa, and the nitride plate part prototyped with test number 4 , 552 MPa was applied at a stress amplitude σa, and fatigue fracture was caused by 8.13 × 10 ⁵ times.

  The observation results are shown in FIG. At fatigue crack initiation positions, typical fatigue fracture surfaces with striations were observed. However, if it is less than 0.05 mm from the shear end face, it exhibits a brittle fracture surface, and the same applies to any specimen. This brittle fatigue fracture surface was confirmed only in a range of less than 0.05 mm from the shear end face in Test No. 6, and in Test No. 20 where the fatigue strength was slightly unsatisfactory, it was slightly over 0.10 mm. It extends. Further, in test number 4 where the fatigue strength was between test numbers 6 and 20, a brittle fatigue fracture surface was observed from a position of approximately 0.075 mm. From this observation result, the fatigue strength is determined by brittle fatigue crack propagation. In particular, the fatigue strength is satisfied by suppressing the brittle fatigue crack propagation region to a range not exceeding 0.10 mm. It is thought that there is a possibility. In the range of less than 0.05 mm, the fatigue strength may be satisfied, but since a brittle fatigue fracture surface is observed, it cannot be a factor satisfying the fatigue strength.

It is thought that nitrogen plays a role of resistance also in the fatigue crack propagation process. Therefore, the relationship between N ^* and fatigue strength was investigated for the purpose of clarifying the relationship between the fatigue crack propagation resistance and fatigue strength in this region, that is, 0.05 mm or more and 0.10 mm or less. .

The result is shown in FIG. In FIG. 6, N ^** does not satisfy the requirement of 0.0600% or more which is the claim of the present invention related to N ^**, and the fatigue strength is satisfied while N ^** satisfies the requirement. Plots are shown using “x” for not satisfied, and “ ^**” for N ^** satisfying the above requirements and satisfying the fatigue strength using a symbol “◯”. First, when N ^** does not satisfy the above requirement, the fatigue strength does not satisfy the purpose regardless of N ^* , which is consistent with the above-described effect. On the other hand, when N ^* is up to 0.4000% by mass, the fatigue strength increases as N ^* increases. However, when N ^* was 1.2,000% or more, it was observed that the fatigue strength tended to decrease as N ^* increased. The lower limit value of N ^* expresses the effect of sticking, and is considered to be a content necessary for suppressing the transition to brittle fatigue crack propagation. On the other hand, when N ^* is excessively high, a high back stress is generated at the moment when it is released from the fixation, and a fatigue crack can be easily propagated.

From the above examination, the relationship between N ^* , N ^** and fatigue strength shown in FIG. 7 was obtained. In the plot of ◆ in FIG. 7, N ^** is less than 0.0600% by mass and does not satisfy the fatigue strength. In the plot of x, N ^* is 0.0600% or more, but the fatigue strength is not satisfied because N ^* is less than 0.4000% or more than 1.2,000%. In the plot of ■, neither N ^* nor N ^** satisfied the fatigue strength. From this result, it is limited to the case where N ^* is 0.4000% or more and less than 1.2,000% by mass, and N ^** is 0.0600% or more by mass. It has been found that it is possible to develop a nitrided plate part that has a fatigue strength equal to or higher than that of the carburized plate part. Note that the upper limit of N ^** is not particularly required, but about 0.7000% is a common-sense upper limit from the nitriding conditions described later. Further, the lower limit of N ^* may be 0.4500% or 0.5000%, and the upper limit of N ^* may be 1.1000% or 1.000%. Further, the lower limit of N ^** may be 0.0650%, 0.0700%, or 0.0800%, and the upper limit of N ^** may be 0.5000% or 0.3000%.

  Further, in the shearing surface of the nitride plate component, the nitrogen content in the central portion in the plate thickness direction of the portion at least 5 mm away from the shear end surface, which is a region where there is no nitrogen intrusion due to gas soft nitriding, is 0.0300% by mass. If it is included in excess, the toughness of the nitrided plate component will be reduced, and it will not function as a component. Moreover, when the nitrogen content of the said part is less than 0.0007% by mass, it will become a very high manufacturing cost in the manufacturing process of the steel plate coil mentioned later. For the above reasons not related to fatigue strength, the nitrogen content in the central portion in the thickness direction of the portion at least 5 mm away from the shear end face is set to a requirement of 0.0007% or more and 0.0300% or less by mass. . The lower limit of the nitrogen content in the central part in the thickness direction of the portion at least 5 mm away from the shear end face is preferably 0.0010%, 0.0015% or 0.0020% by mass%, and the upper limit is 0.0200%. , 0.0100% or 0.0080%.

1.2 Steel plate components excluding nitrogen Next, the reasons for limiting the steel plate components excluding nitrogen will be described. Nitride plate parts generally require a tensile strength of at least 340 MPa in order to achieve both power transmission and weight reduction. On the other hand, if the ductility is lowered (for example, the total elongation of No. 5 test piece of JIS Z2241 is less than 13%), the press formability is hindered in formability and is not suitable for industrial production. In the present invention, in order to achieve both strength and ductility, the following component ranges are assumed. It should be noted that a selection element in a range described later may be included for strength adjustment or the like.

  Except for nitrogen, the elements contained in the nitride plate part of the present invention and the steel plate used therein will be described below. The following components are prerequisites for the present invention in order to exhibit the strength, workability, and the like that are conventionally required for nitride plate parts, including the N content before forming the nitride plate product.

  C content: When C exceeds 0.113% by mass, strength increases and ductility is significantly reduced by forming a pearlite structure. Note that when C is less than 0.025%, the strength is 340 MPa or less, so that the function as a skeletal component of the nitride plate component is not achieved in the first place. In addition, since it will become a peritectic region and slab toughness may fall when C increases, the content of C is preferably 0.10% or less or 0.09% or less. In order to obtain sufficient strength, the C content is 0.034% or more, 0.040% or more, or 0.045% or more.

  Si content: An element that increases the strength as a solid solution strengthening element, but because the pattern resulting from scale spots formed in the finish rolling process remains on the nitride plate part, the wear resistance of the nitride plate part is reduced. In the first place, addition is not preferable. A pattern appears when the mass exceeds 0.10%. There is no need to particularly define the lower limit of the Si content, and the lower limit is 0%. However, if it is less than 0.01%, the raw material cost increases, so the Si content may be 0.01% or more. In order to further improve the wear resistance, the Si content is preferably 0.08% or less.

  P content: Addition exceeding 0.020% by mass reduces press formability and often fails to produce plate parts, and also lowers the toughness of the slab and lowers the productivity of the steel sheet. Accordingly, P is preferably as low as possible, and its lower limit is 0%. However, when the content is less than 0.001%, the manufacturing cost of the steel sheet becomes extremely high. Therefore, the P content may be 0.001% or more. Further, in order to sufficiently ensure plate formability and manufacturability of the steel sheet, the P content is preferably 0.015% or less or 0.013% or less.

  S content: When the content exceeds 0.0200% by mass, a steel sheet containing a large amount of inclusions is produced, and the forming fracture due to press forming becomes significant. Therefore, a low addition amount is preferable, and the lower limit is 0%. However, if it is less than 0.0001%, the manufacturing cost of the steel sheet becomes very high, and there is a concern that the economic effect of the present invention may be lost. Therefore, the S content may be 0.0001% or more. In order to improve press molding, the S content may be 0.0100% or less, 0.0050% or less, or 0.0030% or less.

  Mn content: If the mass is less than 0.71%, the strength is less than 340 MPa, and if it exceeds 1.49%, the ductility is significantly lowered due to the influence of casting segregation. In particular, the performance to nitride plate parts is not adversely affected, but in order to avoid forming a structure extending in the rolling direction due to Mn segregation, the Mn content is 1.40 %% or less, 1.30% or less or It may be 1.25% or less. In order to improve the strength, the Mn content may be 0.75% or more, 0.80% or more, or 0.85% or more.

  Ti content: When Ti exceeds 0.091% by mass, the tensile strength of the steel sheet is increased, and the ductility is remarkably lowered. Therefore, the Ti content is set to 0.091% or less. Further, if Ti is less than 0.020%, the steel sheet does not develop a strength of 340 MPa or more, so it was made 0.020% or more. The lower limit of the Ti content may be 0.025% or 0.030%, and the upper limit may be 0.075% or 0.060%.

  Cr content: an element necessary for imparting wear resistance to nitride plate parts, and it is necessary to add 0.130% or more by mass. On the other hand, if it exceeds 0.340%, the ductility is significantly reduced. For this reason, the upper limit of the Cr content is set to 0.340%. For the effect of wear resistance, the Cr content may be 0.180% or more, 0.200% or more, 0.210% or more, or 0.230% or more. In order to improve ductility, the Cr content may be 0.320% or less or 0.290% or less.

  Al content: It is the minimum necessary element for imparting the wear resistance of the nitride plate parts, and it is necessary to add 0.10% or more. On the other hand, if it exceeds 0.35%, the production cost of the slab becomes very high, so the Al content is made 0.35% or less. In order to improve wear resistance, the lower limit of the Al content may be 0.14% or 0.18%. In order to reduce the slab manufacturing cost, the upper limit may be 0.30% or 0.25%. .

  N content before forming nitrided plate product: Nitrogen before forming nitrided plate product forms coarse nitrides in the manufacturing process of steel plate coil, and if the N content exceeds 0.0100%, the formability of steel plate coil is reduced. Reduce significantly. Therefore, the N content before forming the nitride plate product is preferably as low as possible. However, if it is less than 0.0007%, the cost is extremely high in the manufacturing process of the steel sheet coil, so the lower limit is made 0.0007% or more. In addition, since there is no change in moldability in the range of less than 0.005%, less than 0.005% is preferable for industrial production.

  Here, it is sufficient that the N content before forming the nitrided plate product is an index of press formability before the gas soft nitriding treatment, and therefore, the value may be measured from the plate parts before the gas soft nitriding treatment. Since the plate part is a part before the gas soft nitriding treatment is performed, measurement distortion is caused by the molding strain remaining in the vicinity of the shear end face. Therefore, as described above, the section where the distance from the origin to the center of the plate thickness in the plate thickness direction at a distance of at least 5 mm or more from the arbitrary shear end face in the plate component internal direction is 1 mm along the plate thickness center line from the origin. The average value of N content in the section is obtained. The average value may be measured at any three locations of the plate parts before the gas soft nitriding treatment, and the average value of each value may be adopted. Further, as described later, since the ladle analysis value of the steel material component and the EPMA analysis value are substantially the same value, the ladle analysis value of the steel material component or the emission spectral statement analysis method of JIS G1258 may be used. .

  Further, the following elements are contained within the following range. These elements may be contained in the nitride plate component for a predetermined purpose or as an impurity. The inclusion of these elements is not essential, but the playing element level is also within the range.

  Nb content: Addition exceeding 0.020% by mass reduces the ductility by increasing the tensile strength of the steel sheet, and also forms wrinkles on the surface in the finish rolling step, so the content was made 0.020% or less. . The lower limit is 0.001%, and it is difficult to reduce it to less than this. If the fine-grained structure is desired without affecting the performance of the nitride plate parts, 0.005% or more may be added. In order to improve ductility and prevent surface flaws, the Nb content may be 0.015% by weight or 0.009%.

  Mo content: known as an element that improves the wear resistance of the surface compound layer of the nitride plate component, and may be added to the nitride plate component of the present invention, but if it exceeds 0.140%, the toughness of the slab Decreases the productivity. In order to improve the toughness of the slab, the upper limit may be 0.100%, 0.050%, or 0.010%. The lower limit is 0.001%, and it is difficult to reduce it to less than this.

  V content: Known as an element that improves the wear resistance of the surface compound layer of the nitride plate component, and may be added to the nitride plate component of the present invention, but the lower limit is 0.001%, less than this It is difficult to reduce it in manufacturing. If it exceeds 0.100%, surface flaws will be produced in the finish rolling step described later, and productivity will be impaired. In order to prevent surface flaws, the upper limit may be 0.050%, 0.030%, or 0.010%.

  B content: When bending or flange molding is performed in the press molding process, it may be added in order to improve moldability, but even if it exceeds 0.0030% by mass, the effect is saturated. For this reason, B content shall be 0.0030% or less. The lower limit is 0.0001%, and it is difficult to make it lower than this. In order to improve moldability, the upper limit may be 0.0020%, 0.0010%, or 0.0005%.

  Cu content: Cu does not form compounds with other elements, but precipitates as Cu particles. However, since the Cu particles are precipitated at around 400 ° C., there is no influence on the performance of the nitride plate component. However, in the case of an excessive amount of added Cu, it causes the formation of wrinkles on the surface in the rough rolling step, so the added amount is 0.13% or less. The lower limit is 0.00%. In order to prevent surface flaws, the upper limit may be 0.10% or 0.04%.

  Ni content: Ni is an austenite former element, and when it is excessively added, the toughness of the nitrogen compound formed on the outermost surface of the plate part decreases during nitriding. Therefore, Ni is less than 0.08%. The lower limit is 0.00%. In order to improve toughness, the upper limit may be 0.05% or 0.03%.

  W content: When molten steel containing W solidifies, it forms a very hard eutectic structure and lowers the toughness of the slab. For manufacturability, the addition amount of W is set to 0.07% or less. If necessary, the upper limit of W may be 0.02% or 0.005%. The lower limit is 0.00%.

  Co content: Co, like W, when molten steel solidifies, forms a very hard eutectic structure and lowers the toughness of the slab. For manufacturability, the amount of Co added is 0.07% or less. If necessary, the upper limit of W may be 0.02% or 0.005%. The lower limit is 0.00%.

  Ca content: Ca is an element that improves formability in order to make nonmetallic inclusions fine. However, when the amount of Ca added is 0.0070% or more, the density of nonmetallic inclusions increases. When using Ca, the addition amount is made less than 0.0070%. If necessary, the upper limit of Ca may be 0.0040% or 0.0010%. The lower limit is 0.0000%.

  Mg content: Mg, like Ca, is an element that improves formability in order to make nonmetallic inclusions fine. However, when the added amount of Mg is 0.0050% or more, the density of nonmetallic inclusions increases. When using Mg, the addition amount is less than 0.0050%. If necessary, the upper limit of Mg may be 0.0020% or 0.0008%. The lower limit is 0.0000%.

  REM content: REM, like Ca and Mg, is an element that improves formability in order to make nonmetallic inclusions fine. However, when the amount of REM added is 0.0050% or more, the density of nonmetallic inclusions increases. When REM is used, the amount added is less than 0.0050%. If necessary, the upper limit of REM may be 0.0020% or 0.0005%. The lower limit is 0.0000%.

  Here, “REM” means a rare earth element, more specifically, Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. Any one or more of the above as REM may be contained in the nitride plate component. In addition, content of said REM is content of REM total.

  In addition, in this specification, an impurity is a component which exists in steel regardless of the intention of addition, and does not need to exist originally in the obtained thick steel plate. The term “impurities” is a concept that includes inevitable impurities that are mixed from ores, scraps, or production environments as raw materials when industrially producing steel materials. Such impurities may be included in an amount that does not adversely affect the effects of the present invention.

1.3 Metal Structure Next, as a premise of the present invention, the metal structure that the nitride plate component according to the present embodiment should have is described in order to exhibit the strength and workability that are conventionally required for the nitride plate component. .

  As a premise of the present invention, a steel plate used for manufacturing a nitride plate component is a steel plate having a ferrite fraction of 70% or less in area ratio. If the ferrite fraction is sufficiently low, wrinkle patterns can be prevented from occurring on the surface of the pressed part due to yield elongation. Therefore, the ferrite fraction is set to 70% or less in the metal structure of the nitride plate part. More preferably, the ferrite fraction is 65% or less, 60% or less, or 50% or less.

  The above-mentioned ferrite fraction refers to the area ratio of the ferrite structure in the metal structure. The area ratio of the ferrite structure is a value measured with a test piece which is taken from a position at a quarter thickness away from the surface of the steel plate or from the center of the plate thickness and subjected to nital corrosion after mirror polishing. This metal structure is taken with an optical microscope at a magnification of 200 times or more and 1000 times or less, and an image of three or more fields of view may be taken at each plate thickness position. For all images, the area ratio of ferrite in the metal structure is obtained, and the average value of the area ratio of ferrite in all images is defined as the ferrite fraction of the steel sheet.

  The metal structure of the nitride plate component is mainly composed of ferrite and bainite. For this reason, the total area ratio of ferrite and bainite may be 50% or more, preferably 60% or more or 65% or more while satisfying the area ratio of the ferrite. In addition to ferrite and bainite, pearlite, martensite, austenite, and the like may be present.

2. Next, a method for manufacturing a nitride plate component according to the present invention will be described. That is, the limited range of the manufacturing method for controlling the above-described N ^* and N ^** within the target range will be clarified. The following describes the reason for the limitation of the steel plate components and the steel plate manufacturing method excluding nitrogen as the range that satisfies the minimum role on the industrial product as nitride plate parts, and then manufactured to control the nitrogen content to the limited range. The method will be described in detail.

The method for manufacturing a nitride plate component according to the present invention includes:
Chemical composition is mass%,
C: 0.025% or more, 0.113% or less,
Si: 0.10% or less,
Mn: 0.71% or more, 1.49% or less P: 0.020% or less,
S: 0.0200% or less,
Ti: 0.020% or more, 0.091% or less,
Cr: 0.130% or more, 0.340% or less,
Al: 0.10% or more, 0.35% or less,
N: 0.0007% or more, 0.0100% or less,
Nb: 0.001% or more and 0.020% or less,
Mo: 0.001% or more and 0.140% or less,
V: 0.001% to 0.100%,
B: 0.0001% or more and 0.0030% or less,
Cu: 0.00% or more and 0.13% or less,
Ni: 0.00% or more and less than 0.08%,
W: 0.00% or more and 0.07% or less,
Co: 0.00% or more and 0.07% or less,
Ca: 0.0000% or more and less than 0.0070%,
Mg: 0.0000% or more and less than 0.0050%,
REM: 0.0000% or more and less than 0.0050%, and balance: slab containing Fe and impurities is hot rolled in a range of hot rolling finish rolling outlet temperature of 850 ° C. or more and less than 960 ° C. Get
Thereafter, cooling is started within 3 seconds after the hot rolling finish rolling is completed, and further, the steel sheet is cooled to 460 ° C. or more and 630 ° C. or less within 29 seconds after the hot rolling finish rolling is finished,
By winding up the steel sheet, the area ratio of the ferrite structure in the metal structure is a steel sheet coil of 70% or less,
Furthermore, in the case of manufacturing a nitride plate part using the pickled steel plate coil, after unrolling the steel plate coil, the steel plate has a plastic strain amount of 0.03% or more and 3.00% or less. Of bending and unbending,
Without re-rolling the steel plate again, it is subjected to shearing and press forming into a plate part shape,
In a closed furnace adjusted to a temperature of 500 ° C. or higher and lower than 620 ° C. in an atmosphere having a volume composition ratio of more than 30% ammonia gas, the steel sheet is allowed to stay for nitriding for 60 minutes or longer.

  Hereinafter, as a premise of the present invention, conditions for manufacturing a steel sheet coil having strength and workability that are conventionally required for nitride plate parts will be described. That is, in order to achieve both strength and ductility, as well as providing a limited range of components, the steel sheet coil manufacturing method is applied with plastic strain to the steel sheet surface layer described below, except for subsequent rewinding, nitriding conditions, The conditions for hot rolling are based on the following condition ranges so as not to adversely affect the production of the nitrided plate parts. The reason for limiting the chemical composition of the slab is the same as the reason for limiting the chemical composition of the nitride plate component described above, and thus the description thereof is omitted.

2.1 Hot Rolling / Cooling First, the slab is hot-rolled in a temperature range of 850 ° C. or more and less than 960 ° C. to obtain a steel plate. Here, when the hot rolling finish rolling outlet temperature is higher than 850 ° C., the slab deformation resistance at a high temperature is increased, and the load of the rolling roll at the time of finish rolling becomes extremely high, which is not suitable for industrial production. On the other hand, if the hot rolling finish rolling temperature is 960 ° C. or less, the crystal grains become coarse, and the ductility of the steel sheet is lowered. The hot rolling finish rolling outlet temperature is preferably 885 ° C or higher or 895 ° C or higher. Further, the hot rolling finish rolling outlet temperature is preferably less than 950 ° C or less than 940 ° C.

  Next, cooling is started within 3 seconds after hot rolling finish rolling. When the time from finish rolling to the start of cooling exceeds 3 seconds, the crystal grains become coarse so that the ductility of the steel sheet is lowered and the elongation is less than 13%.

In the same cooling, the steel sheet is cooled to 460 ° C. or more and 630 ° C. or less within 29 seconds after hot rolling finish rolling. Here, when the cooling stop temperature is less than 460 ° C., the strength of the steel sheet is remarkably increased, the ductility is further lowered, and the elongation is less than 13% at worst. The cooling stop temperature is preferably 490 ° C. or higher, more preferably 510 ° C. or higher. On the other hand, if the cooling stop temperature is higher than 630 ° C., the ferrite fraction is higher than 70%, which leads to the occurrence of yield point elongation, wrinkles are generated, and the crystal grains are coarsened. Is further reduced, and the elongation is less than 13% at worst. Moreover, the ferrite fraction of the steel plate obtained can fully be reduced as cooling stop temperature is 630 degrees C or less. The cooling stop temperature is preferably 590 ° C. or lower, more preferably 560 ° C. or lower.
Furthermore, when the time from the end of hot rolling to the end of cooling exceeds 29 seconds, the crystal grains become coarse, so that the ductility of the steel sheet is further reduced, and the worst is less than 13%. The time from finish rolling to the cooling stop temperature is preferably 25 seconds or less, more preferably 22 seconds or less.
Then, the obtained steel plate is wound up and pickled.

2.2 Bending / Unbending and Shearing / Press Forming Next, the amount of plastic strain applied to the steel sheet coil, which is the main requirement of the present invention, whether or not the steel sheet coil is rewound after applying the amount of plastic strain, and gas soft nitriding treatment The conditions will be described below.

About the steel plate coil pickled, after unrolling the steel plate coil, the steel plate is subjected to bending and unbending in the range of 0.03% or more and 3.00% or less in the amount of plastic strain, without rewinding the steel plate again, Shear processing and press molding are performed to form plate parts. Below, in the manufacturing method of the nitride plate component using the steel plate coil mentioned above, a process required in order to control N ^* and N ^** which are requirements of this invention to a limited range, and the limited range are described in detail.

In the process of clarifying the above N ^* and N ^** requirements, the test numbers 12, 13, 17 and 21 in Tables 2 and 3 were not subjected to bending / unbending correction. there were. These were no exception, and N ^* did not satisfy the lower limit value, and the fatigue strength was not satisfied. On the other hand, all of the test numbers 14, 19, 20, and 22 in which the shape was extremely bad and a strong strain was applied in the straightening process of bending and unbending showed a high N ^* result.

  Therefore, the influence of the amount of plastic strain in the aforementioned bending / unbending process was investigated. In the investigation, the steel plate coils O, Q and T in Table 1 were used to unroll the steel plate coil, and the plastic strain amount was changed by bending and unbending deformation with different roll diameters. Here, the plastic strain is measured by drawing a 2 mm grid pattern in advance on the surface of the steel sheet, and measuring the nominal strain measured from the change in shape of the grid pattern before and after bending and bending deformation. This strain is the amount caused by permanent deformation. Therefore, this is adopted as the plastic strain amount as it is. A test was also performed on a sample that was given a predetermined amount of plastic strain and then rewound as a steel plate coil.

FIG. 8 shows the influence of the amount of plastic strain on N ^* . The open symbol in FIG. 8 is obtained by applying a predetermined plastic strain in the leveler process and then moving directly to the shearing process. In addition, the solid symbol represents a state in which a predetermined plastic strain is applied in a bending / unbending process, the steel sheet is rewound into a coil shape, the steel sheet coil is rewound again, and then the process is transferred to the shearing process. . In addition, in each sample in FIG. 8, the gas soft nitriding process was performed on the conditions of volume composition ratio of ammonia 50%, temperature 560-575 degreeC, and processing time 90-150 minutes. In addition, after applying a predetermined plastic strain in the bending / bending process, and then moving to the shearing process as it is, if the amount of plastic strain exceeds 3.0%, N ^* exceeds 1.20% by mass. The result does not depend on the steel sheet coil. On the other hand, when the plastic strain amount was less than 0.03%, N ^* was less than 0.4000% by mass. In addition, after giving a plastic strain, what re-rolled the steel plate again in the coil shape became N ^* less than 0.4000% irrespective of the amount of plastic strain.

Next, the influence of plastic strain on N ^** is shown in FIG. Each sample in FIG. 9 is also subjected to gas soft nitriding under the conditions that the volume fraction of ammonia is 50%, the temperature is 560 to 575 ° C., and the treatment time is 90 to 150 minutes. In FIG. 9, when the plastic strain exceeds 3.00%, N ^** is less than 0.0600% by mass. There was no difference in N ^** between the case where a predetermined plastic strain was applied in the bending / bending process and then the process was transferred to the shearing process, and the case where it was not. These results are thought to be due to the dislocation state of the steel sheet coil. That is, when the plastic strain is high, the frequency of stationary dislocations is high, and surface nitrogen intrusion is excessive during gas soft nitriding. On the other hand, when the plastic strain is low or when the steel sheet coil is rewound again, movable dislocation is introduced. At this time, since the atomic vacancies in the steel sheet are not nitrogen but are consumed for the increase of movable dislocations, it is considered that the penetration of nitrogen is inhibited. Although it is extremely difficult to distinguish between movable dislocations and immovable dislocations, since the dislocation state is specific to the amount of plastic strain, the amount of plastic strain ranges from 0.03% to 3.00%. After giving the bending / unbending, the limiting condition was to apply shearing and press forming into a plate part shape without rewinding the steel sheet again.

Note that N ^* in the range of 0.05% or more and 1.50% or less of the plastic strain amount is a substantially constant value regardless of the plastic strain amount. From the viewpoint of industrial management, the amount of plastic strain is preferably 0.05% or more and 1.50% or less.

  By the way, in the manufacturing process of a steel plate, skin pass rolling may be performed for the purpose of removing yield elongation. In this step, rolling is performed for the purpose of introducing plastic strain into the steel plate. At this time, the amount of plastic strain is extremely small so as not to remove the ductility of the steel sheet. In such a skin pass, the roll reduction and the tension in the longitudinal direction of the plate are adjusted to obtain a predetermined amount of plastic strain. In other words, it undergoes deformation in which the position just below the reduction and the extended position coincide. For this reason, the vicinity of the surface is subjected to strong friction, and the dislocation distribution is unique in the vicinity of the surface layer. As an invention utilizing this, Patent Document 3 discloses an example in which the fatigue strength is improved by controlling the dislocation density distribution of 50 μm from the surface layer, making the steel plate composition appropriate, and increasing the maximum penetration depth of nitrogen. ing. In order to confirm whether or not the same mechanism contributes also in the bending and bending back process described above, the inventors described that the method described in the same document is within 50 μm in the sheet thickness direction from the steel sheet surface before nitriding. The ratio of the dislocation density at the 1/4 position in the plate thickness direction was investigated. The results are listed in Table 4. As a result, in the bending and unbending processes, a change in the dislocation density ratio due to the amount of plastic strain is recognized, but a dislocation density ratio of 2.0 times or more cannot be obtained. This is considered because the above-described bending / bending return step is a step without roll reduction and friction caused thereby.

That is, the reason why the characteristics of dislocation density in Patent Document 3 are not obtained is considered to be because the skin pass rolling with a rolling reduction of 0.5% to 5% and F / T ≧ 80000 is not performed. The “F” is a linear load (Kg / mm) obtained by dividing the rolling mill load by the steel plate width, and the “T” is a load per unit area (Kg / mm) applied in the longitudinal direction of the steel plate. ² ) are shown respectively.

Thus, even if the ratio of dislocation density is less than 2.0 times, sufficient fatigue strength is satisfied. Therefore, the improvement of the fatigue strength of the nitride plate component in the present invention is not due to the dislocation density of the steel sheet. I can say that. Furthermore, the technique described in Patent Document 3 is a method for controlling the maximum curing depth, but N ^** , which is the point of the present invention, cannot be controlled. This is because the higher the dislocation density in the vicinity of the surface layer, the more nitrogen is accumulated in the vicinity of the surface layer, and the amount of nitrogen diffused from the surface layer to a deep position is reduced. Therefore, the fatigue strength of nitride plate parts is not satisfactory in the first place.

  Moreover, about a shearing process and press molding, it is not specifically limited, It can carry out suitably by the method known to those skilled in the art.

2.3 Gas soft nitriding treatment Finally, in an atmosphere having a volume composition ratio of more than 30% ammonia gas, shearing was performed for 60 minutes or longer in a closed furnace adjusted to a temperature of 500 ° C. or higher and lower than 620 ° C. The processed and press-formed steel sheet stays and is nitrided. Thus, a nitride plate component can be obtained.

The reasons for limiting the gas soft nitriding conditions that satisfy the nitrogen content of the nitride plate parts will be described below. First, when gas soft nitriding is performed in an atmosphere with a volume composition ratio of 30% or less of ammonia gas, the nitrogen supplied to the pressed parts is reduced and N ^* does not exceed 0.4000% by mass. ^** does not exceed 0.0600% by mass. The volume composition ratio of ammonia gas in the atmosphere may be more than 30%, but is preferably 40% or more. The volume composition ratio of ammonia gas in the atmosphere is preferably 65% or less, and preferably 55% or less.

Further, even when the treatment temperature is less than 500 ° C., the decomposition reaction of ammonia gas is suppressed, and N ^* does not become 0.4000% or more. On the other hand, when the processing temperature is 620 ° C. or higher, the growth of the surface compound layer becomes dominant, and as a result, N ^** does not become 0.0600% or higher by mass. The treatment temperature is preferably 520 ° C. or higher, more preferably 540 ° C. or higher. The treatment temperature is preferably 600 ° C. or lower, more preferably 580 ° C. or lower.

Further, when the nitriding time is less than 60 minutes, the diffusion time is short and N ^** does not become 0.0600% or more by mass. Note that N ^** can be increased when the treatment time is long, but the gas soft nitriding treatment cost is increased. If it is preferably within the range of 270 minutes or less, both economic efficiency and durability of the gas soft nitriding plate part can be achieved. The treatment time may be 60 minutes or longer, but preferably 90 minutes or longer. The above is the feature of the product of the present invention and the reason for limiting the manufacturing method.

  Next, examples of the present invention will be described. The following embodiments are merely examples of the present invention, and the present invention is not limited to the following embodiments.

A nitriding plate component having the shape of FIG. 1 was prototyped by the manufacturing method shown in Table 4 using slabs in the range of components of Try 1 to 11 shown in Table 1. In Tables 4 and 5, “Q”, “O”, and “T” are the same steel plate coils as “Q”, “O”, and “T” described in Tables 2 and 3, and are nitride plates. Only the position in the longitudinal direction of the steel sheet used for manufacturing the parts is different. Therefore, the cooling stop temperature (CT) is slightly different from the temperatures described in Tables 2 and 3. In the fatigue test of the prototyped nitride plate part, a stress amplitude which was not repeatedly broken up to 10 ⁷ times was defined as fatigue strength by applying a repeated load with a frequency of 25 Hz and a stress ratio of 1. As the stress value, a strain gauge was attached to the gray position in FIG. 3 in the circumferential direction, and the measured value was adopted. The results are shown in Table 5. In addition, each symbol in Tables 4 and 5 represents the same meaning as the symbols described in Tables 2 and 3. The acceptance / rejection has a threshold value of the above-mentioned fatigue strength of 517 MPa or more.
The “thickness center N amount at a position 5 mm from the shear end face” in Table 5 as a region where there is no nitrogen intrusion due to gas soft nitriding is the thickness center in the part 5 mm away from the shear end face in the normal direction. Measuring the nitrogen content at three locations at 0.003 mm intervals from the origin along the thickness center line from the origin is performed at three arbitrary shear end faces, and the average value of the measurement results is shown in Table 5 did. Except for the measurement of the nitrogen content, the chemical composition was not analyzed at a position 0.6 mm or more away from the surface of the nitrided plate part including the shear end face as a region where nitrogen intrusion by gas soft nitriding was not caused. If the ladle analysis result of the steel material used in 1 is regarded as the analysis result of the chemical composition at a position separated by 0.6 mm or more, the ladle analysis value of the nitrogen content of the steel material in Table 1 and The analytical values of the nitrogen content of the nitride plate parts are almost the same. From this, it can be confirmed that there is no nitrogen intrusion due to gas soft nitriding when the distance from the surface is 0.6 mm or more. In addition, regarding the nitrogen content, the chemical composition analysis results (such as ladle analysis values) of the steel plates used for the nitride plate parts are located at a distance of 0.6 mm or more from the surface of the nitride plate parts including the shear end face. It can also be confirmed that it may be considered. It is also possible to use the emission spectral statement analysis method of JIS G1258.

As a result of the fatigue test, the fatigue strength was the result of the carburized plate parts or more only in the test numbers 24 to 27, 31 to 33, 37 to 39, 45 to 48, 51 and 52 satisfying N ^* and N ^** . In Test Nos. 23, 30 and 36, the amount of plastic strain was less than 0.03%, and N ^* was less than 0.4000% by mass. On the other hand, the test numbers 28, 34 and 40 in which the plastic strain amount exceeded 3.00% had N ^* exceeding 1.2,000% by mass without exception. Furthermore, even if the plastic strain amount is 0.03% or more and less than 3.00%, N ^* is a mass in the test numbers 29, 35, 41, 53, 54 and 55 in which the coil is rewound after the strain is applied. It became less than 0.7000%. In test numbers 42 and 43 in which the ammonia gas ratio is 30% or less, N ^* is less than 0.7% by mass, and N ^** is less than 0.0600% by mass. In test number 49 where the treatment temperature was less than 500 ° C., N ^* was less than 0.4% by mass, and in test number 50 where the treatment temperature was 620 ° C. or more, N ^** was less than 0.0600% by mass. Further, in test number 44 with a processing time of 50 minutes, N ^** was less than 0.0600% by mass. From the above, the validity of the requirements of the present invention was verified.

In addition, the present inventors experimentally confirmed that it is difficult to make N ^* and N ^** within the above-described range by the methods described in Patent Documents 1 and 3.

  The preferred embodiments of the present invention have been described in detail above with reference to the accompanying drawings, but the present invention is not limited to such examples. It is obvious that a person having ordinary knowledge in the technical field to which the present invention pertains can come up with various changes or modifications within the scope of the technical idea described in the claims. Of course, it is understood that these also belong to the technical scope of the present invention.

Claims (2)

A nitride plate part having a shear end face,
The chemical composition excluding nitrogen is mass%,
C: 0.025% or more, 0.113% or less,
Si: 0.10% or less,
Mn: 0.71% or more, 1.49% or less P: 0.020% or less,
S: 0.0200% or less,
Ti: 0.020% or more, 0.091% or less,
Cr: 0.130% or more, 0.340% or less,
Al: 0.10% or more, 0.35% or less,
Nb: 0.001% or more and 0.020% or less,
Mo: 0.001% or more and 0.140% or less,
V: 0.001% to 0.100%,
B: 0.0001% or more and 0.0030% or less,
Cu: 0.00% or more and 0.13% or less,
Ni: 0.00% or more and less than 0.08%,
W: 0.00% or more and 0.07% or less,
Co: 0.00% or more and 0.07% or less,
Ca: 0.0000% or more and less than 0.0070%,
Mg: 0.0000% or more and less than 0.0050%,
In the nitride plate component containing REM: 0.0000% or more and less than 0.0050%, and the balance: Fe and impurities, and the area ratio of the ferrite structure in the metal structure is 70% or less,
The nitrogen average content in the range of 0.05 mm or more and 0.10 mm or less in the distance from the shear end face to the shear end face normal direction of the nitride plate part is 0.4000% or more and 1.2000% or less in mass%, and The minimum nitrogen content of 0.015 mm or more and 0.200 mm or less is 0.0600% or more,
Nitrogen content in the region where no nitrogen intrusion occurs due to nitriding treatment, the origin is the thickness direction center of the nitride plate part at least 5 mm away from the shear end face in the normal direction of the shear end face, and the thickness center line from the origin A nitrogen plate component in which the nitrogen content is measured on a line at 3 to 5 points in a section up to a position 1 mm away along the line, and the average value of the nitrogen content is 0.0007% or more and 0.0300% or less. Chemical composition is mass%,
C: 0.025% or more, 0.113% or less,
Si: 0.10% or less,
Mn: 0.71% or more, 1.49% or less P: 0.020% or less,
S: 0.0200% or less,
Ti: 0.020% or more, 0.091% or less,
Cr: 0.130% or more, 0.340% or less,
Al: 0.10% or more, 0.35% or less,
N: 0.0007% or more, 0.0100% or less,
Nb: 0.001% or more and 0.020% or less,
Mo: 0.001% or more and 0.140% or less,
V: 0.001% to 0.100%,
B: 0.0001% or more and 0.0030% or less,
Cu: 0.00% or more and 0.13% or less,
Ni: 0.00% or more and less than 0.08%,
W: 0.00% or more and 0.07% or less,
Co: 0.00% or more and 0.07% or less,
Ca: 0.0000% or more and less than 0.0070%,
Mg: 0.0000% or more and less than 0.0050%,
REM: 0.0000% or more and less than 0.0050%, and balance: slab containing Fe and impurities is hot rolled in a range of hot rolling finish rolling outlet temperature of 850 ° C. or more and less than 960 ° C. Get
Thereafter, cooling is started within 3 seconds after the hot rolling finish rolling is completed, and further, the steel sheet is cooled to 460 ° C. or more and 630 ° C. or less within 29 seconds after the hot rolling finish rolling is finished,
By winding up the steel sheet, the area ratio of the ferrite structure in the metal structure is a steel sheet coil of 70% or less,
Furthermore, in the case of manufacturing a nitride plate part using the pickled steel plate coil, after unrolling the steel plate coil, the steel plate has a plastic strain amount of 0.03% or more and 3.00% or less. Of bending and unbending,
Without re-rolling the steel plate again, it is subjected to shearing and press forming into a plate part shape,
A nitride plate in which the steel sheet is allowed to stay in a closed furnace adjusted to a temperature of 500 ° C. or higher and lower than 620 ° C. for 60 minutes or longer in an atmosphere having a volume composition ratio of ammonia gas of more than 30% and nitrided. A manufacturing method for parts.